NEW YORK—Artist Jasper Johns delivered what prosecutors hope will be a masterpiece in their prosecution of a one-time successful foundry owner on charges he tried to sell a bronze sculpture of Johns’ iconic 1960 “Flag” painting for $11 million and then copied works of two other artists for tens of thousands of dollars more.

The 83-year-old artist, whose works have sold for millions of dollars, testified Thursday for more than an hour in federal court in Manhattan, saying repeatedly that he never authorized Brian Ramnarine to make a bronze copy of the sculpt-metal painting “Flag” when he asked him in 1990 to make a wax mould of it.

Johns was the star witness for prosecutors trying to prove Ramnarine tried to sell an unauthorized bronze sculpture of the painting in 2010. Four bronze copies of “Flag” were made in 1960, and the jury was shown a picture of President John F. Kennedy posing with one that was given to him.

The Sharon, Conn., artist said he was considering making a gold sculpture from a wax mould when he chose Ramnarine to create the mould because his work for him in the past had been “excellent as far as I was concerned.”

He said he took the wax copy to his home, then in Manhattan, and put it in the refrigerator so it wouldn’t be damaged by heat. Ramnarine, he said, never returned the mould, as was a customary practice by most foundries.

The white-haired Johns seemed clear about his memory of his dealings with Ramnarine, who has pleaded not guilty, and that a “Flag” sculpture with his name on it that was offered as an exhibit in the trial was fake, along with his signature on the back of it.

Johns said that not long after he made a verbal agreement with Ramnarine to create the wax mould with a payment in cash, a relative of Ramnarine came to his studio and showed him an unauthorized bronze sculpture of “Flag.”

Shown the exhibit, he said he recognized the “X” through his “fake signature” that he marked on it when he was shown it in the early 1990s.

“It is my work, but it is not mine,” he said. “It’s finished in a way that I would not have finished it. The frame is smooth.”

He said he recalled having written in ink on the back of it that it was not his work.

“That seems to have been removed from it,” Johns said.

Prosecutors showed Johns another bronze copy of “Flag” that he also said was a fake, though the signature appeared to be a photocopy of his real signature.

The artist also testified that documents with his signature at the bottom certifying that a bronze copy of “Flag” was real were also fakes.

“No, I had nothing to do with this document,” he said of one of them.

Assistant U.S. Attorney Zachary Feingold asked if he ever gave to Ramnarine a copy of a painting that was sold to the Metropolitan Museum of Art for millions of dollars.

“No,” Johns said with a chuckle.

He said he didn’t believe he had spoken with Ramnarine since 1990 and wasn’t friends with him.

Defence lawyer Troy Smith tried to establish through his cross examination that a former assistant to Johns who has been charged with selling 22 works stolen from Johns’ Connecticut studio may have authorized Ramnarine to make bronze copies of “Flag.”

Johns acknowledged that he sent the longtime assistant to Ramnarine’s business to try to retrieve the mould.

Smith told Johns he had no knowledge of what the assistant told Ramnarine.